On the "terrain pool" (or rather, "territory pool"); instead of randomizing each round's selections, how about the person who picked first in one round, picks last in the next, and vice versa? That's how most CCG drafts etc are run. It balances out better and would go quicker.
Also, you could record the order each territory gets picked in, and use that as the basis for a points system for territory selection by people who join your ongoing campaign. For example, if a territory gets picked in the first round, its worth 5 points; if picked in the last round, its worth 1 point. This is modified a bit by the order they are picked in; the first territory picked each round is worth an extra .4; the last one picked is worth .4 less. If you have 5 or more players picking, the second one picked could be worth an extra .2, and the second to last worth .2 less.
Then, when you want to allow people to join your ongoing campaign, give them points to buy territories with, rather than having them roll. The obvious number would be 15 points, but since they get the advantage of picking the exact ones they want, you might want to drop that to 14, 13.5, or something like that. You could even allow them to purchase more or less than 5 territories, perhaps at the cost of giving up a point.